Glass fiber for optical telecommunication purposes are generally covered with a synthetic resin coating to prevent mechanical damage. To prevent optical transmission losses caused by microbends a coating consisting of various layers is preferred. For example, the following method is used: Immediately after the glass fiber has been formed, for example by drawing from a preform or using the double crucible method, a first, soft buffer layer of a synthetic rubber is applied having a modulus of elasticity of from 0.1 to 10 MPa. In order to protect the soft buffer layer during further treatments to which the optical fiber is subjected, a second, harder top layer of a synthetic resin is provided having a modulus of elasticity which exceeds 100 MPa. This top layer is also provided immediately after the glass fiber has been formed, i.e. before the fiber is led over a guide wheel or stored. The buffer layer and the top layer together form the primary synthetic resin coating of the glass fiber. A customary value for the diameter of the glass fiber having a primary synthetic resin coating is 250 .mu.m, the diameter of the glass fiber being 125 .mu.m.
To protect the optical fiber from ambient influences during cabling, during layer of the cables and during the life of the cables, the optical fiber is often additionally provided with a thicker secondary synthetic resin coating having a modulus of elasticity exceeding 1 GPa. This secondary synthetic resin coating is not necessarily applied directly after the formation of the glass fiber.
Two embodiments of such a secondary synthetic resin coating are used. In one embodiment, the optical fiber with the primary synthetic resin coating is arranged so as to lie clear off the secondary synthetic resin coating which, hence, forms a tube. The space between the optical fiber and the tube is generally filled with a thixotropic liquid or gel, for example a silicone oil filled with very small quartz particles. It is also possible to accommodate accomodate more than one optical fiber in the tube. In the other embodiment the secondary synthetic resin coating is bonded in an adhesive manner to the primary synthetic resin coating.
In European Patent Application No. EP 155051 which corresponds substantially to U.S. Pat. No. 4,718,748, issued Jan. 12, 1988 and assigned to U.S. Phillips Corporation, a description is given of an optical fiber which is provided with a synthetic resin coating which is formed from a curable synthetic resin composition which is cured by exposure to radiation. The primary synthetic resin coating consists of two layers having moduli of elasticity of from 0.1 to 10 MPa and 100 MPa, respectively. Both layers each have a thickness of approximately 30 .mu.m.